Spatio-temporal Pattern of Cells Expressing the Clock Genes Period and Timeless and the Lineages of Period Expressing Neurons in the Embryonic CNS of Drosophila Melanogaster

The initial steps towards the generation of cell diversity in the central nervous system of the fruitfly Drosophila melanogaster take place during early phases of embryonic development when a stereotypic population of neural progenitor cells (neuroblasts and midline precursors) is formed in a precise spatial and temporal pattern, and subsequently expresses a particular sequence of genes. The clarification of the positional, temporal and molecular features of the individual progenitor cells in the nerve cord and brain as well as of their specific types of neuronal and/or glial progeny cells forms an essential basis to understand the mechanisms controlling their development. The present study contributes to this effort by tracing the expression of period and timeless, two genes that encode transcription factors with a key role in the molecular mechanism of the biological clock. Using a combination of genetic markers and immunocytochemistry with antibodies specific for period and timeless we define the number, location, origin and lineage of period cells in the nerve cord throughout embryogenesis. We also provide the first description of the expression of timeless in the embryonic central nervous system. We found a major transformation in the number and types of cells that express period and timeless takes place between embryonic and larval life.